It is very unclear at this point what a valid use case for this feature
would possibly be. The old documentation only mentions $(hostname) as
an example, which can be done with $HOSTNAME instead.

Note that $(...) is still supported in Exec lines of desktop files,
this does not require [$e] anyway (and actually works better without it,
otherwise the $ signs need to be doubled to obey kconfig $e escaping rules...).

Hehe, that's what I did initially, and the value being read was (hostname) without the $ because of the way [$e] works. A bit surprising, but in line with the fact that $/ $? $@ etc would also remove the $ (because the code just sees an empty env var name), and if someone wanted to keep the $ they would have to write $$. So I concluded invalid testcase, nobody would write this anymore. But OK, it's a test about old files that might have this. I'll re-add the test.